In one form of the known screen printing method, for the stepwise printing of successive patterns or images on a web of material, the web is carried on a moving support, below the screen printing stencil with its associated squeegee or doctor. The stencil is lowered on to the material for the printing operation during which the stencil is moved in the direction of movement of the web, from an initial starting position to a final or end position relative to the stationary doctor. While the stencil is moving in this way, the doctor presses print medium e.g. ink or dye through the stencil on to the web, whereafter the stencil and doctor are lifted from the web and are returned to their relative starting positions.
It will be appreciated that with this process there is the disadvantage that the material to be printed must be fed forward with a stepwise movement with the stencil so that, irrespective of whether the patterns or images to be printed are applied to the web, at spacings from each other or in direct succession, that is to say, without any space between each two consecutive images, only a relatively low throughput of print material can be achieved.
In another form of screen printing method, use is made of a stencil in the form of an endless moving belt driven synchronously with the web which is passed continuously through the printing mechanism. With this process and apparatus the images to be printed may be printed on the web without any spacing between each two consecutive images, and very high levels of material through-put can be achieved. However, for certain cases of printing, this process suffers from the disadvantage that the stencil belt provides virtually no possibility of adjustment as regards the distribution of the printed patterns or images on the web, that is to say, the pattern repeat length. This is because the length of the stencil belt is fixed by the distance between the two guide rollers or drums around which the stencil belt is passed. It will indeed rarely happen that, with a given pattern repeat length, that pattern repeat length or a multiple thereof is to be precisely disposed on the screen printing stencil, that is to say, without any residual sections or portions which are shorter than a pattern length. Theoretically it might be possible to adapt the length of the stencil belt to particular circumstances, as by corresponding adjustment of the distances between the rollers or cylinders around which the stencil belt passes, but this presupposes extremely complicated and thus expensive mechanisms for this purpose. Another disadvantage of this is that the production costs of a stencil belt of this kind are so high that such a stencil belt is only used when a very large number of items are to be printed. An example in this connection is the production of labels or tickets which are generally produced by first printing on a web of material and then separating the individual labels or tickets from the printed web. If a large issue of a given ticket or label is to be produced, it is quite possible to use the above-mentioned method, but if smaller numbers are to be issued, then the production costs per ticket or label are excessively high.